Artificial recharge to a freshwater-sensitive brackish-water sand aquifer, Norfolk, Virginia

Fresh water was injected into a brackish-water sand for storage and retrieval. The initial injection rate of 400 gpm decreased to 70 gpm during test 3. The specific capacity of the well decreased also, from 15.4 to 0.93 gpm. Current-meter surveys indicated uniform reduction in hydraulic conductivity of all contributing zones in the aquifer. Hydraulic and chemical data indicate this was caused by dispersion of the interstitial clay upon introduction of the calcium bicarbonate water into the sodium chloride bearing sand aquifer. The clay dispersion also caused particulate rearrangement and clogging of well screen. A pre-flush of 0.2 N calcium chloride solution injected in front of the fresh water at the start of test 4 stabilized the clay. However, it did not reverse the particulate clogging that permanently reduced permeability and caused sanding during redevelopment. Clogging can be prevented by stabilization of the clay using commercially available trivalent aluminum compounds. Test 1 and test 2 showed that 85 percent of the water injected can be recovered, and the water meets U.S. Public Health Standards. Storage of fresh water in a brackish-water aquifer appears feasible provided proper control measures are used. (Woodard-USGS)